The present disclosure relates to a thin film transistor (TFT), a method of manufacturing the thin film transistor, a display, and an electronic apparatus.
It is known that an oxide (an oxide semiconductor) of zinc (Zn), indium (In), gallium (Ga), tin (Sn), or a mixture thereof exhibits excellent semiconductor property. For example, a thin film transistor using an oxide semiconductor exhibits electron mobility ten or more times that of a thin film transistor using amorphous silicon, and exhibits favorable off characteristics. Therefore, it is expected that the thin film transistor using an oxide semiconductor is applied to a drive device for a liquid crystal display or an organic electro luminescence (EL) display with a large screen, high definition, and a high frame rate (see Japanese Unexamined Patent Application Publication No. 2010-016163).
However, the oxide semiconductor has insufficient heat resistance. Thus, oxygen desorption occurs due to heat treatment or plasma treatment in processes of manufacturing the thin film transistor, and lattice defect is accordingly formed. The lattice defect causes formation of electrically shallow impurity level and reduction in resistance of the oxide semiconductor. Therefore, when the oxide semiconductor is used in an active layer, a threshold voltage is decreased due to increase in defect level, and leakage current is increased. Accordingly, so-called depression operation in which a drain current flows even when a gate current is not applied may occur. Further, if the defect level is continuously increased, the operation is shifted from a transistor operation to a conductor operation. It is assumed to be due to change in stability depending on content ratio of thermally-unstable elements particularly in the case of a multi-elemental oxide semiconductor. Moreover, hydrogen has been reported as an element forming shallow impurity level in addition to the lattice defect (“n-type doping of oxides by hydrogen”, Cetin Kilic, et al., APPLIED PHYSICS LETTERS, Jul. 1, 2002, Vol. 81, No. 1, pp. 73-75).
In addition, oxide semiconductors such as zinc oxide and indium gallium zinc oxide (IGZO) have excellent semiconductor property, and application to a thin film transistor (TFT) and the like is under consideration. The thin film transistor using an oxide semiconductor has large electron mobility and excellent electric characteristics compared with a thin film transistor using amorphous silicon. Moreover, the thin film transistor using an oxide semiconductor has favorable off characteristics, and is expected to have high mobility even at temperature near room temperature.
However, the oxide semiconductor has insufficient heat resistance. Thus, when the oxide semiconductor is applied to a thin film transistor, desorption of oxygen, zinc, and the like occurs to form lattice defect in a heat treatment process in manufacturing the thin film transistor. The lattice defect causes formation of electrically shallow impurity level and reduction in resistance of the oxide semiconductor.
Therefore, it is proposed that a protection layer formed of an insulating material is provided on an oxide semiconductor layer to suppress desorption of oxygen and the like from the oxide semiconductor layer (for example, see Japanese Unexamined Patent Application Publication No. 2008-60419).